[strong] Adding regression cases which should always continue working.

1) Mutually recursive functions
2) Strong code using non-strong var before declaration.

These test cases ensure that we don't accidentally kill these wanted behaviors
when adding restrictions to strong mode.

BUG=

Review URL: https://codereview.chromium.org/927143002

Cr-Commit-Position: refs/heads/master@{#26671}

test/mjsunit/strong/mutually-recursive-funcs.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --strong-mode
+
+"use strong";
+
+function foo(param, fooCount, barCount) {
+  if (param === 0)
+    return {'foo': fooCount, 'bar': barCount};
+  return bar(param - 1, fooCount + 1, barCount);
+}
+
+function bar(param, fooCount, barCount) {
+  if (param === 0)
+    return {'foo': fooCount, 'bar': barCount};
+  return foo(param - 1, fooCount, barCount + 1);
+}
+
+(function TestMutuallyRecursiveFunctions() {
+  let obj = foo(10, 0, 0);
+  assertEquals(obj.foo, 5);
+  assertEquals(obj.bar, 5);
+})();

test/mjsunit/strong/use-before-var.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --strong-mode
+
+function strongFunction() {
+  "use strong";
+  // This should not be a "use before declaration" error, even though the
+  // variable is defined later (by non-strong code). Rationale: Non-strong code
+  // is allowed to use var declarations, and the semantics of var declarations
+  // say that the variable should be usable before the declaration.
+  return notStrong + 123;
+}
+
+var notStrong = 456;
+
+(function TestStrongFunctionUsingLaterDefinedNonStrongVar() {
+  assertEquals(strongFunction(), 579);
+})();